Due to the effect of a spatial context or temporal context, the perceived orientation of a test line or grating pattern can appear tilted away from its physical orientation. The tilt illusion (TI) is the phenomenon that the perceived orientation of a test line or grating is altered by the presence of surrounding lines or grating with a different orientation (spatial context; see Fig.1). And the tilt aftereffect (TAE) is the phenomenon that the perceived orientation is changed after prolonged inspection of another oriented line or grating (temporal context; see Fig.2). It has been reported that the magnitude and the direction of the perceived orientation shift depends on the relative orientation between test and contextual stimuli (see Fig.3). Psychophysics experiments have shown that relative orientations between 0 deg and about 50 deg produce repulsion effects (the test line or grating tends to rotate away from the contextual stimulus), which is known as the direct form of the tilt effect; but larger relative orientations up to 90 deg produce attraction effects (the test line or grating tends to rotate towards the contextual stimulus), which is known as the indirect form of the tilt effect. It has been observed repeatedly that indirect effects are smaller than direct effects. The repulsion peak is about 3 degrees usually when the relative orientation between the test and contextual stimuli is around 20 degrees; and the attraction peak is usually maximally 0.5 degrees when the relative orientation is around 70 degrees (see Fig.3).
The original experiments showing the TI and TAE These effects were first studied by Gibson in 1937. The subject's vision was restricted so that he could see a black line (the test line) bisecting a white circular field, and he could grasp the edges of a disk to rotate the line about its midpoint. An experimenter would sit behind the disk to set the stimuli and to record the subject's adjusted position of the line. During the tilt aftereffect experiment, the subject was required to look at an oriented line for four minutes, and then to adjust another line to a position which appeared to be vertical. In the simultaneous tilt illusion experiment, a tilted grating was introduced into the circular field of the subject, and the subject was supposed to set the adjustable line to vertical before and after the tilted grating had been superposed on it. Both experiments showed that the position which appeared to follow the subject's perceived vertical was slightly off the objective vertical, and the perceived orientation shifts depended on the relative orientation between the test line and the adapted line or the simultaneously induced line.
Tilt effects under various conditions The tilt effects have been tested with various stimulus parameters, such as spatial frequency, color, luminance and contrast differences between the test grating and the contextual grating, and disparity depth or temporal separation between them. Dichoptic presentation, "invisible" and natural image contextual stimuli have also been studied. It has been shown that both the TAE and the TI are spatial frequency specific, since both effects (TI and TAE) of the direct form (repulsion) are reduced considerably if the test and the contextual grating differ in spatial frequency. It has been further suggested by Wenderoth and Johnstone (1988) that separation between the contextual and test stimuli, with either the spatial gap or the spatial frequency difference, reduces the magnitude of the direct but not the indirect tilt illusion. They also showed that reducing the diameter of the contextual stimulus reduces the direct effect but the indirect effects are relatively constant. According to Durant's paper in 2006, in the direct form of tilt effects, the largest illusion occurs when the test stimulus and the context surround are presented simultaneously; the spatial gap, the relative contrast and depth cues result in a reduced TI. Experiments also show that both TI and TAE occur for contextual and test stimuli that differ in color and luminance. When the test line is presented in one eye and the context in the other (dichoptic presentation), the magnitude of the tilt illusion reduces, suggesting that at least part of the effect is due to monocular cells. And a reversed tilt effect was observed very recently: a direct form (repulsion) of TI under monocular presentation becomes indirect (attraction) for dichoptic stimulation, when the vertical test line inclined by a 20 deg line. Another interesting experiment was conducted by Clifford and Harris (2005), in which the contextual surround was followed immediately by a random noise mask covering the surround but not the center, so the contextual surround would not be consciously perceived. It turned out that an oriented contextual grating can affect the perceived orientation of the test grating even outside of awareness of this context. Furthermore, the illusion maintains when contextual textures have a broad range of orientations (e.g. natural images), even those without a clearly perceivable orientation; other oriented features, including illusory contours, an ellipse, a moving dot and a row of dots or lines, also can induce a robust tilt illusion.
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